The present invention relates to a thermal spraying powder, a thermal spray coating obtained from the thermal spraying powder, and a hearth roll including the thermal spray coating obtained from the thermal spraying powder.
A roll for conveying a steel plate called a hearth roll is disposed in a heat treatment furnace such as a steel plate continuous annealing furnace. A steel plate is subjected to heat treatment in a furnace maintained under a reduction atmosphere of N2/H2 or the like. At that time, a deposition called a buildup is formed on the surface of the hearth roll by a reaction of the roll with the steel plate in some cases. When a buildup is formed on the surface of the hearth roll, a pressed scar or the like is formed on the surface of a steel plate conveyed on the hearth roll, thereby resulting in poor quality of the steel plate. Therefore, when a buildup is formed on the surface of the hearth roll, it is necessary that the operation of the furnace be immediately stopped and the surface of the hearth roll be cleaned, so that production efficiency is remarkably reduced. Accordingly, buildup formation has been conventionally prevented by providing a thermal spray coating on the surface of the hearth roll.
Meanwhile, in recent years, the demand for high tension steel has increased. The high tension steel contains elements such as manganese (Mn) and silicon (Si) as solid solution reinforcing elements in an amount larger than that of normal steel. Since these elements are easily oxidized, a layer enriched in oxides of these elements is formed on the surface of a high tension steel plate. Since a manganese enriched layer particularly tends to form a buildup by reacting with a thermal spray coating provided on the surface of a hearth roll, this manganese buildup has caused a problem in a hearth roll for conveying a high tension steel plate. As the required quality of a steel plate has become increasingly strict, a problem of the buildup has become increasingly apparent. Therefore, development of a thermal spraying powder aiming such a thermal spray coating as to solve these problems has been conducted (see, for example, Japanese Laid-Open Patent Publication Nos. 2005-206863 and 2003-27204).
Particularly high buildup resistance is required for a thermal spray coating provided on the surface of a hearth roll used in a high temperature zone (for example, 900° C. or more) in a furnace. At the same time, high thermal shock resistance which can resist without causing separation by thermal shock accompanied by, for example, passing a steel plate therethrough is also required for such a thermal spray coating. However, a thermal spray coating for satisfying these requirements has not yet been obtained under the present circumstances.